Variable frequency inverter drive systems are increasingly used for driving electrical machines. A typical drive has two or more stages of power conversion, including a rectifier to convert AC power to a constant or variable DC voltage, and an inverter for the conversion of the DC voltage to AC power of variable frequency and controlled voltage to provide the desired excitation to the electric machine. Improvements in semiconductor power devices have enabled significant improvements in such drives, as has the availability of faster microprocessors and digital signal processors (DSPs). Variable frequency drives are of particular importance for driving large industrial motors and traction motors for vehicles. The most common types of variable frequency drives have been current source inverters, conventional six-step voltage source inverters, or cycloconverters (particularly for low speed operation). Such traditional approaches have generally suffered from the limitations of high device voltage ratings and relatively poor output waveform quality, with a substantial harmonic content present in the output waveform. The availability of improved high speed semiconductor switching devices has stimulated consideration of other inverter configurations that enable the synthesizing of output voltages with lower distortion and improved waveform quality.
Several inverter topologies have been proposed to allow the realization of multilevel output voltage waveforms that more closely replicate the desired fundamental sine wave. Such approaches include, e.g., the flying capacitor topology, J. S. Lai, et al., "Multilevel Converters--A New Breed of Power Converters," Proc. IEEE-IAS' 95 Conf., pp. 2441-2548; the use of cascaded H-bridges, which has been successfully implemented in commercially available large drives (3000-4000 HP) and some static VAR compensators, F. Z. Peng, et al., "A Multilevel Voltage Source Inverter with Separate DC Sources for Static VAR Generation," Proc. IEEE-IAS' 95 Conf., pp. 2541-2548; and the so-called "Vienna" rectifier topology which uses only three active switches, J. W. Kolar, et al., "Current Handling Capability of a Neutral Point of a Three-Phase/Switch/Level Boost-Type PWM (Vienna) Rectifier," Proc. IEEE-PESC' 96 Vol. 2, pp. 1329-1336. Partially active rectifier topologies have also been demonstrated, Y. Phao, et al., "Forced Commutated Three-Level Boost Rectifier," IEEE Trans. on Industry Applications, Vol. 31, No. 1, January/February 1995, pp. 151-161.
Another type of multilevel drive configuration, based on a neutral point clamped inverter topology, is described in a paper by A. Nabae, et al., "A New Neutral Point Clamped PWM Inverter," IEEE Trans. on Industry Applications, Vol. IA-17, No. 5, September/October 1981, pp. 518-523. See also, U.S. Pat. Nos. 4,135,235, 4,167,775 and 4,220,988 to Baker and Baker, et al. The neutral point clamped (NPC) topology is an extension of the conventional voltage source inverter but with the possibility of doubling the voltage rating because the clamp diodes serve to limit the voltage stress on each device. A three level NPC converter is capable of producing 19 voltage vectors as opposed to the 7 vectors of conventional voltage source inverters. The NPC inverter provides "redundant switching states" because there are multiple means of generating some of the voltage vectors. The greater number of voltage vectors implies that, for a constant device switching frequency, the voltage total harmonic distortion from the NPC inverter should be lower than for a conventional voltage source inverter. Multilevel inverters of this type also have additional advantages that are very significant in high power applications. One example is that the dv/dt stresses on the switching devices are reduced and, in addition, the lower voltage steps at each switching event reduces the wear and tear on the winding insulation, thus improving long-term reliability. Reduced voltage waveform distortion reduces the output filtering requirements, resulting in additional cost savings. The NPC inverter topology can be implemented in inverters having greater than three levels, which are commonly referred to as diode clamped multilevel inverters (DCMLI). Because of the inherently higher voltage capabilities of such inverters, they have initially found applications in power utilities as static VAR compensators.
For a given system voltage rating and number of levels, it can be shown that the DCMLI topology has the minimum possible total active switch rating, a very significant performance and cost component of high power drives. While three level inverters have been common, four level and higher level inverter systems have also been proposed; see, e.g., Gautam Sinha, et al., "A Four Level Rectifier-Inverter System for Drive Applications," Proc. IEEE-IAS' 96 Conf., 1996, pp. 980-987.
Multilevel drives use two or more DC bus capacitors connected in series across the outermost DC bus lines from the rectifier, with the multilevel inverter connected to receive the DC power across each of the capacitors, each of which forms one of the voltage levels in the output voltage waveform. Unequal power draw from the capacitors can lead to unequal voltages across the capacitors, and consequent deviation of the output voltage waveform from the desired waveform. Three level inverters have been reported which overcome the capacitor voltage balancing problem in various ways, including utilizing inverter switching strategies based on look-up table values, Jie Zhang, "High Performance Control of a 3 Level IGBT Inverter Fed AC Drive," Proc. IEEE-IAS' 95 Conf., 1995, pp. 22-28; by the addition of zero sequence components, J. K. Steinke, "Control Strategy for a Three Phase AC Traction Drive with Three Level GTO PWM Inverter," Proc. IEEE-PESC' 88 Conf., 1988, pp. 431-438; and through a combination of space vector control and redundant state selections, A. Nabae, et al., supra. While such schemes have been utilized for three level inverters, they are not readily extended to four and higher level inverters. Four level inverters have the advantage of permitting the use of smaller, hence lower cost, devices, and offer the possibility of being more directly connected to high voltage supply lines since each device is required to switch a smaller fraction of the total supply voltage. However, unbalances in the capacitor voltages can produce unequal device stresses in the inverter, which can lead to device failure as well as degradation of waveform quality.